Those skilled in the art will, of course, appreciate that a typical well logging measurement involves using a density logging tool to determine what is commonly known as the bulk density of the earth formations traversed by a borehole. Generally these formation bulk density measurements are correlated with other measurements obtained from sonic, resistivity or pulsed neutron logs to calculate formation properties and characteristics of interest. To obtain these bulk density measurements, the body of the logging tool is typically decentralized in the borehole so that one face of the tool is urged into sliding engagement with one side of the borehole. One or more gamma energy detectors in the tool body will then measure the attenuation of gamma energy emitted from a source of gamma energy in the tool body after this energy has passed through a predetermined path in the adjacent earth formations. Standard computational techniques which are well known in the art are then employed to use these attenuation measurements for determining the formation bulk densities of the formations that are being logged.
Heretofore, the typical bulk density logging tool has been provided with a single arm which is pivotally mounted on the rear of the tool body and operatively arranged to be biased outwardly against one wall of the borehole so as to urge the forward face of the tool body into sliding engagement with the opposite wall of the borehole. Typically, this pivoted arm is also coupled to a measuring device so that as the outward end of the arm is moved inwardly and outwardly in accordance with irregularities on the borehole walls, the measuring device will provide a series of output measurements representative of the transverse dimensions of the borehole between the outward end of the arm and the forward face of the tool body.
It will be appreciated, however, that variations in the cross-sectional shape or configuration of a borehole will significantly affect the measurements of the diameter of that borehole. Ideally, a borehole should have a circular cross-sectional shape. In practice, however, boreholes frequently have generally-oval cross-sectional shapes due to wobbling of the drill bit or keyseating by the drill string. Also, it is common for a borehole to have so-called "caves" in the sides of the borehole at different depth intervals in the borehole. Thus, in situations such as these, measurements of the lineal distance between the forward face of the tool body and the outward end of this pivoted arm are not representative of the precise diameter of the borehole along an assumed diametrical axis. Accordingly, in a borehole which has an irregular or non-circular cross-section, it is entirely possible that the logging tool is angularly oriented in such a manner that the transverse distance being measured is a geometrical chord at some depth intervals of the borehole. Thus, heretofore, it has been necessary either to assume that these measurements provided by a pivoted decentralizing arm are sufficiently accurate for a given logging operation or else to rely upon making any critical calipering measurements with a different type of calipering tool.